The present invention relates to a common mode filter for PWM motor drives and other types of power electronic equipment. A conventional passive common mode filter, illustrated schematically in FIG. 1, is undesirably large and expensive. This is particularly so for motor drives, where the amplitude of the common mode current and the corresponding size of the passive common mode filter increases with increasing length of the motor cable. For example, with a cable length of 100 meters, the size and cost of the passive common mode filter can be comparable to that of the PWM drive itself. With reference to FIG. 1, the common mode current pulses drawn by the PWM drive are shunted via filter capacitors designated Y-cap and blocked from the network by the common mode inductor L. As shown in FIG. 1, a conventional common mode filter employs a common mode inductor L having a typically toroidal core C having a winding wound on the core for each line, here L1 and L2. Each line is coupled to ground GND by a filter capacitor Y-cap. The common mode current drawn by the PWM drive or other equipment and flowing in the ground-line from the PWM drive is circulated through the Y-caps. The resulting common mode current thus does not appear on the lines L1 and L2 or the ground line GND as conducted EMI. In order to provide this filtering action, the capacitors and inductors are of substantial size and thus increase the size and cost of the equipment.
With reference to FIG. 1, undesired high frequency components of the common mode current drawn by the drive are shunted by the Y-caps. A typical maximum value for the Y-cap in an industrial drive is 0.1 uF. The value of the common mode inductance L is set by the frequency spectrum of the common mode current drawn by the drive, and the attenuation of conducted EMI that must be achieved at the input side of the filter. Although the circuit of FIG. 1 shows the filter in the AC line coupled to the PWM drive, the filter may also be disposed in, for example, the DC bus of a power converter, between the rectifier and the inverter.
The operation of the conventional passive filter is illustrated by the waveforms in FIG. 2, which show the calculated response of the filter to a given wave shape of common mode current drawn by the drive. The effect of the applied line voltage is substantially irrelevant to this discussion, and is ignored here. FIG. 2(a) shows the equivalent circuit and FIG. 2(b) shows the waveforms of the circuit of FIG. 1.
At t0, a first positive pulse of common mode current, of 5A amplitude and pulse width of 0.5 uS, is drawn by the drive. At the end of this current pulse, the voltage, ec, across the Y-cap has risen to above 16V. By time t1, when a negative pulse of common mode current is drawn by the drive, ec has already swung negative, due to resonant discharge through L, with corresponding current IL through L. The negative pulse of common mode current drawn by the drive at t1 now further increases the negative voltage on the Y-cap.
At t2, ec has swung back into the positive direction, and is now further boosted by the positive pulse of common mode current drawn by the drive at this time. After several cycles, the voltage ec and the current iL attain relative high xe2x80x9csteady statexe2x80x9d levels.
The build-up of voltage across the capacitor/inductor illustrated by the waveforms in FIG. 2 arises because the resonant frequency of the filter is close to the PWM switching frequency of the inverter of the PWM drive. Of course, the filter would normally be designed so that resonance or near-resonance at the PWM frequency is avoided.
A design where the resonant frequency of the filter is significantly higher than the PWM frequency is exemplified in FIG. 3, and one where it is significantly lower, in FIG. 4. As shown, in either case the inductor still has significant voltage applied during the periods between inverter switching events. (But note the scale change between these Figs. and FIG. 2).
The inductor must be sized to support the total voltage-time integral impressed across it. This voltage-time integral determines the maximum flux in the core. The required size (and cost) of the core are approximately proportional to this integral. The vast majority of the voltage-time integral is generated during intervals between inverter switching events, when common-mode current is not drawn by the drive. Thus the inductor has to handle a much greater voltage-time integral than is impressed just during the short conduction periods of the common mode current drawn by the drive.
A further undesirable feature of the conventional filter is that although the passive filter attenuates higher frequency components of common mode current, in the range above 150 kHz, it actually amplifies the lower frequency components that are related to the lower order harmonics of the PWM frequency.
The present invention provides a method for substantially reducing the size and cost of a conventional passive common mode filter, by using a switch in series with the xe2x80x9cYxe2x80x9d capacitors of the common mode filter. This allows a reduction in the size of the core of the common mode inductor by up to an estimated 90-95%. The switch is controlled in synchronism with the inverter switching instants, so that the common mode voltage impressed by the Y-caps across the inductor during the periods between inverter switching instants is substantially eliminated by switching the capacitor out of the circuit except for during the time when common mode current is drawn by the equipment. This permits a very significant reduction in the size of the inductor.
The invention accordingly comprises an electromagnetic interference filter for filtering common mode current in an inverter device, the filter comprising an inductor coupled in a least one power supply line to the inverter device; the inverter device having a ground return line, a capacitor coupled between the inductor and the ground return line; and a controlled switch coupled in series with the capacitor between the inductor and ground return line, and a control unit controlled in accordance with commutations in said inverter device whereby said switch is turned on when common mode current is drawn by said inverter device and turned off when common mode current ceases substantially to be drawn by said inverter device.
The invention also comprises a method of filtering electromagnetic interference due to common mode current in an inverter device, the method comprising: switching common mode current pulses in a supply line including an inductor to the inverter device through a capacitor coupled between the supply line and a ground return line for the inverter device only when common mode current is drawn by the inverter device.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.